The present invention relates to cellular telephone systems. More specifically, the present invention relates to a system for increasing the reliability of the cellular telephone system in environments having substantial multipath propagation or under conditions wherein a large number of mobile telephone units simultaneously attempt to access a base station.
Many communications systems have multiple transmitters that need to randomly access one or more receivers. A local area network (LAN) is one example of such a multiaccess system. A cellular telephone system is another. In any such system, when several transmitters attempt to transmit simultaneously, the messages may interfere or "collide" with one another. A receiver cannot distinguish among the messages involved in the collision.
Two such multiaccess protocols, commonly called the "Aloha" and "Slotted Aloha" protocols, are described in Bertsekas et al., Data Networks, chapter 4, Prentice-Hall, Englewood Cliffs, 1987. In the Aloha protocol, each transmitter may transmit a message at any time. Upon discovering that the transmitted message has collided, the transmitter waits a random delay time and retransmits the message. In Slotted Aloha, all messages fit into a time slot of a predetermined length. Upon discovering that the transmitted message has collided, the transmitter delays a random number of slots and then retransmits the message. In both methods, a random delay is introduced to prevent transmitters from retransmitting simultaneously.
The use of code division multiple access (CDMA) modulation is one of several techniques for facilitating communications in which a large number of system users are present. The use of CDMA techniques in a cellular telephone system is disclosed in U.S. Pat. No. 5,056,031 entitled "Method and Apparatus for Controlling Transmission Power in a CDMA Cellular Telephone System" and in U.S. patent application Ser. No. 07/543,496 entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System" now U.S. Pat. No. 5,109,398, both assigned to the assignee of the present invention and incorporated herein by reference.
In the above-mentioned patent, a multiple access technique is disclosed where a large number of mobile stations, each having a transceiver, communicate through base stations, also known as cell-sites, using CDMA spread spectrum communication signals. The base stations are connected to a mobile telephone switching office (MTSO), which in turn is connected to the public switched telephone network (PSTN).
The use of CDMA spread-spectrum techniques maximizes the number of mobile stations that can communicate simultaneously with the base station because the same frequency band is common to all stations. Each mobile has a pseudonoise (PN) code uniquely associated with it that the mobile station uses to spread its transmitted signal. In the above-referenced patent, this PN code is called the "long PN code." Once the call has been initiated, i.e., the base station has selected the long PN code corresponding to the transmitting mobile station, the base station can receive and de-spread the signal transmitted by the mobile station. Similarly, the mobile station can receive and de-spread the signal transmitted by the base station. In some systems, the signals may be modulated with a "pilot" PN code as well.
However, for certain types of transmissions, it is advantageous to use a common PN long code, rather than a unique long code for each mobile station. The message transmitted by a mobile station attempting to initiate a call is one example of such a transmission. A mobile station wishing to initiate calls can transmit such requests on a common "access channel" using a corresponding common PN code. The base station can monitor the access channel by despreading the signal using this PN code. The access channel is used because messages such as those for initiating a call are relatively short in comparison to voice transmissions, and a receiver could more easily monitor a relatively few access channels than the large number of unique "traffic channels" with which the mobile stations are associated by their unique PN long codes.
The access channel may be used by the mobile station not only to initiate a call, but to transmit any information to the base station at a time other than during a call that has already been initiated. For example, the access channel may be used by the mobile station to respond to an incoming call initiated by a base station over a "paging channel."
Under any of the conditions discussed above, multiple mobile stations may transmit simultaneously on the access channel. When two mobile stations transmit simultaneously and there is no multipath, the transmissions arrive at the base station separated in time by a delay equal to the difference of twice the distance between each mobile station and the base station. Under most operating conditions, it is unlikely that a large number of mobile stations will be at precisely equal distances from the base stations. However, simultaneously transmitted messages would collide if two or more stations are at the same range. Under most conditions, the base station can distinguish among the transmissions because the time between arrivals of the transmissions at the base station exceeds one PN chip.
Some operating conditions tend to produce collisions. Collisions are likely to occur when a large number of mobile stations approach the edge of a cell simultaneously, a condition causing handoffs of the mobile stations. The access channel transmissions arrive at the base station simultaneously because the mobile stations are at substantially the same distance from the base station when at the edge of the cell.
It is also possible that a large number of mobile users would attempt to simultaneously initiate calls for other reasons such as following a natural disaster. The simultaneous transmissions of multiple mobile stations on the access channel may exceed the maximum throughput of the processor in the base station.
The probability of access channel collisions increases with an increase in the number of mobile stations and with an increase in multipath reflections. Multipath compounds the problem because, while the main signals of two transmissions may be separated in time by more than one chip, multipath components of the transmissions may not be. Furthermore, as discussed in copending U.S. patent application Ser. No. 07/432,552 entitled "Diversity Receiver in a CDMA Cellular Mobile Telephone System," filed Nov. 7, 1989, now U.S. Pat. No. 5,109,390, a base station diversity receiver may have multiple correlators that combine received multipath components to improve message quality. However, ambiguities may exist between multipath components that would reduce the effectiveness of the diversity receiver. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.